It is well known in the art to nondestructively inspect the internal quality of articles of produce, such as apples, by transmitting light through the apples and analyzing the emerging light. A number of different mechanical and optical systems have been proposed for performing this nondestructive internal inspection. See for example, U.S. Department of Agriculture Technical Bulletin No. 1341, "The Difference Meter for Measuring Internal Quality of Foods and Biological Tissues," by G. S. Birth and K. N. Norris, issued September 1965, and an article, "On-Line System Sorts Fruit on Basis of Internal Quality," by R. D. Rosenthal and D. R. Wester, published in Food Technology, July 1973.
Because transmitted light emerging from a dense article of produce often is at a low level, it is extremely important that the light detector be shielded from ambient light and from light that tends to randomly bounce around the article and into the photodetector. Effective light shielding means have presented an obstacle to the industry in its attempt to provide a practical, high volume, commercial internal quality produce grader.